Distance measuring instrument for pedicle screws

ABSTRACT

The invention relates to a distance measuring instrument for pedicle screws which detects the spacing between two pedicle screws ( 3, 3′ ) with the ends ( 26, 27 ) of two crossing limbs ( 11, 12 ) and displays it on a crossbar ( 25 ) fixedly connected to one of the limbs at a scale ( 22 ). Since the second limb ( 12 ) is divided into a pointer ( 16   a ) and into a flexural spring extending parallel to it at which a pronounced thumb grip ( 21   a ) is fastened, and since the thumb grip is connected to a scale ( 24 ) via which a pre-determined pre-tension can be read off relative to the pointer ( 16   a ), a spacing measurement can take place under a controlled pre-tension.

The invention relates to a distance measuring instrument for pediclescrews having two limbs which cross at an axis of rotation and whoseends can be positioned at two pedicle screws screwed into adjacentspinal vertebrae, with a first limb merging at the opposite side to itsend into a crossbar which is provided with an arc-shaped scale part,with the second limb having a projection projecting over the crossbarfor adjustment and being formed as a pointer to indicate the spacingbetween the pedicle screws with the pointer at a scale of the scalepart. Such a spacing measurement serves for the defining of the lengthof components which are intended to bridge the distance between the twopedicle screws.

The company Centerpulse Orthopedics Ltd. (Altgasse 44, CH-6340 Baar) hasa spacing measurement device in its instruments such as is shown in FIG.2 as prior art. Such a measurement device allows the spacing between twoscrewed in pedicle screws to be measured when the screw heads are easilyvisible and the contact of the ends can thus be visually inspected. Whenthe heads of the pedicle screws are not directly visible, a spacingmeasurement is very difficult.

It is the object of the present invention to improve this condition withrespect to operating techniques which only provide for a small, minimuminvasive operation field. This object is satisfied by the characterisingpart of claim 1 in that the second limb is divided into a pointer and aflexural spring extending parallel to it, at which a pronounced thumbgrip is fastened; and in that the crossbar is shaped as a handle toproduce a spreading force at the ends via the flexural spring which canbe read off relative to the pointer via a scale connected to the thumbgrip.

This arrangement has the advantage that a tactile feedback for thecontacting of the ends is present at the thumb grip during the readingof the spacing of the ends under pre-tension. It has furthermore beenfound that an initial yielding between two vertebrae can also be takeninto account by a pre-determined pre-tension force on spreading todetermine the installation length of a supporting element.

Advantageous further developments of the invention are shown by thefeatures of dependent claims 2 to 10.

A robust and less susceptible design for the spacing measurement deviceresults when the second limb with pointer and flexural spring is guidedat both sides in an elongate slot of the crossbar. In the arc-shapedscale part, a likewise arc-shaped groove for a key is worked in which isdesigned as a trailing pointer and is carried along by the pointer atthe second limb at a projecting dog. The pointer actually connected tothe key remains at the scale of the arc-shaped scale part as a storedvalue, irrespective of whether the pointer of the second limb has toreduce its spacing, for example in order to move centring elements outof centring bores at the pedicle screws.

The tactile feedback with respect to the spacing position reachedbetween two pedicle screws can be substantially improved if matchingcentring devices are attached to the ends of the limbs which centre inthe spreading direction at the head of a pedicle screw or at elementsconnected thereto. The end of a limb can thus be formed in the spreadingdirection with a nose in the form of a projecting hemisphere or of acone stub in order to be centred in a bore of the head. As long as sucha limb is under pre-tension in the spreading direction, the centringelements cannot slide off. At the same time, by a light shaking at thelimb, the tactile feedback is given that the end is centred at itsprovided position. A centring can also be carried out at the emerging ofthe band or cable with a limb end which is fork-shaped even with pediclescrews where a cable has already been drawn in the head.

It is furthermore advantageous to crank the ends of the two limbs in theform of laterally offset projections such that the limbs themselves andthe rest of the distance measuring instrument lie laterally offsetrelative to the plane set up by the two pedicle screws. This allowstools and centring elements to be used such as are described in aparallel application in an independent manner because they are arrangedspatially offset. If instruments or centring parts are used which engageperpendicularly from above at the head of a pedicle screw which ishardly visible, then the ends of the limbs can additionally have aguiding fork in the spreading direction with which the ends are guidedon their way to the head until an actual centring at the head ispossible.

Further advantages in handling arise when the scale for the reading ofthe pre-tension is attached to a hook which engages behind the pointerwith clearance to prevent over-stressing of the flexural spring and toread off the pre-tension within this clearance. If the limbs of themeasuring device are made of metal (e.g. of a non-rusting steel alloy)and if the crossbar, which guides the second limb, is made of plastic, alow inherent weight and a favourable and silent material pairing for theguide results. It is furthermore possible to provide a whole set ofdifferent ends pushable onto the limbs as adapter pieces which have afork shape in the direction of the limb axis, a centring sphere or coneshape in the spreading direction, a lateral offset and/or a fork shapein the spreading direction.

The invention will be described with reference to embodiments in thefollowing. There are shown:

FIG. 1: schematically, a section with two pedicle screws screwed intoadjacent vertebrae;

FIG. 2: schematically, a view of a known spacing measurement device forthe pedicle screws of FIG. 1;

FIG. 3: schematically, a side view of a distance measuring instrument inaccordance with the invention;

FIG. 4: schematically, a side view of a further distance measuringinstrument analogue to FIG. 3;

FIG. 5: schematically, a view obliquely from above of the distancemeasuring instrument of FIG. 4;

FIG. 6: schematically, a section at the end of a second limb whichcontacts at its offset end a pedicle screw with a screwed in centringpart;

FIG. 7: schematically, an end of the second limb of FIG. 6 with ahemispherical projection for the centring at a passage bore of thepedicle screw;

FIG. 8: schematically, a section at the end of a first limb which at itsoffset end, which is centred as a fork at a cable drawn through thepedicle screw, contacts the head of the pedicle screw; and

FIG. 9: schematically, a very enlarged section of the scale part and thedisplays of the distance measuring instrument of FIG. 4.

FIGS. 1 and 2 show how a spacing measurement has previously been madebetween two pedicle screws. Two pedicle screws 3, 3′ are screwed intotwo adjacent spinal vertebrae 1, 2 which are separated by anintervertebral disc 9. Each of the pedicle screws 3, 3′ has a head 4,with end faces 5, at which bridging parts are later pushed. The heads 4are aligned such that their passage bores 31 are flush. There arenotches 10 at the side at the heads 4 at which further instruments canbe aligned. A previously known distance measuring instrument consists oftwo limbs 11, 12 which cross at an axis of rotation 13 and whose ends26, 27 can be spread, for example to detect the spacing between the twopedicle screws 3, 3′ and to read it off with the second limb 12 as thepointer 16 at an arc-shaped scale 22 which is attached to a crossbar 25of the first limb 11. The pointer 16 is moved with a projection 21 forthis purpose.

A first example of the invention is shown in FIG. 3. A distancemeasuring instrument contacts two adjacent vertebrae 1, 2 with pediclescrews 3, 3′ at its limbs 11, 12. The first limb 11 has a fork-shapedend 26 with which it contacts the pedicle screw 3′ and with which it issimultaneously centred at a cable 7 projecting from the head of thepedicle screw. The cable 7 is fixed in a passage bore 31 by a clampingscrew 6. The fork-shaped end 26 contacts the head of the pedicle screw3′ at a slight pre-tension. The second pedicle screw 3 likewise has apassage bore 31 in its head 4 into which the cable 7 is drawn at a latertime. The second limb 12 is centred at this passage bore 31 at its end27 which has a projection projecting in the spreading direction.

The two limbs 11, 12 cross at an axis of rotation 13. A cross bar 25made of plastic is screwed to the first limb 11 opposite to its end 26and first projects laterally as a scale part 18 with a curvature in theform of an arc of a circle with the axis of rotation 13 as the centrepoint and merges into a handle 20. The two limbs 11, 12 are made ofmetal, for example of a non-rust steel. The first limb is fastened tothe crossbar 25 by screws 19. The second limb 12 is guided in anelongate slot 32 of the crossbar 25 and split in its longitudinaldirection into two components, a pointer 16 a and a flexural spring 15which are both captured in the elongate slot 32. The flexural spring 15extends parallel to the pointer 16 a, is fastened to the second limb 12by screws (not shown), for example, and is fixedly connected at itsupper end to a thumb grip 21 a projecting above the crossbar 25. Thepointer 16 a has at its upper end an arrow marking 23 which indicatesthe spacing of the two ends 26, 27 on a scale 22 of the arc-shaped scalepart 18. When the handle 20 is gripped by the hand and the flexuralspring 15 is drawn towards the hand by the thumb grip 21 a, apre-tension arises at the ends 26, 27 of the limbs 11, 12 whichcorresponds in a tactile manner to a feedback on the contacting of theends 26, 27 at the pedicle screws 3, 3′, with the spacing of the endsbeing able to be read off at a pre-determined pre-tension because ascale 24 has been attached to the thumb grip 21 a at which thepre-tension can be read off relative to the arrow marking 23 of thepointer 16 a. An advantage of the device consists of the fact that itcan be operated with one hand and leaves the second hand free for thesurgeon for additional manipulations such as the holding back of tissueparts standing in the way.

A further embodiment is shown in FIGS. 4 to 9 in which furtherimprovements to the embodiment of FIG. 3 are included. The samereference numerals have been used as in FIG. 3. The elongate slot 32(FIGS. 5 and 9) divides the curved scale part 18 of the crossbar 25 intoa front part 44 and into a rear part 43. A continuous, curved groove 33has been applied in the front part 44 and a key 14, which itself has afriction brake, is displaceably supported in this in order to be takenalong in the spreading direction as a trailing pointer 17 by the pointer16 a. For this purpose (FIG. 9), a dog 45 is attached to the key 14which projects into the elongate slot 32 and is taken along by thepointer 16 a. The actual key 14 is, for example, inserted from the sideof the elongate slot 32 and secured from the front by a trailing pointer17 screwed on as a securing plate. On the drawing of the thumb grip 21a, the trailing thermometer 17 is taken along by the pointer 16 a. Ifthe ends 26, 27 now meet the resistance of the pedicle screws, theflexural spring can be pre-tensioned so much via the thumb grip until apre-determined tension has been reached. At this tension, the largestspacing also occurs between the two pedicle screws 3, 3′. On thesubsequent relief of the thumb grip, the trailing pointer remains at theposition of the largest spacing and thus stores the measured valuewithout any influencing taking place on the further moving together ofthe tips 26, 27, for example for the releasing of centring projections36 from the passage bores 31 of pedicle screws.

The scale 24 for the reading of the pre-tension is attached to a hook 28bent out of the thumb grip 21 a which is partly guided around thepointer 16 a in order to avoid over-straining of the flexural spring 15in that the hook 28 abuts the pointer 16 a. This means that the pointer16 a must be so stable that it is also not plastically deformed onnon-professional handling of the thumb grip 21 a. In FIGS. 6, 7 and 8,alternatives are shown in the region of the ends 26, 27 of the limbs 11,12. Both limbs 11 and 12 are cranked toward the end and have projections29, 30 offset laterally relative to the spreading direction which eachform the end. The lateral offset has the advantage that the wholedistance measuring instrument is arranged laterally offset relative tothe pedicle screws and does not take up the space directly above thepedicle screws for itself. Furthermore, the projections 29, 30 haveopened guiding forks 34, 35 in the spreading direction with which theycan be guided from the outside up to the head of the pedicle screw at aslight pre-tension along centring parts 39 or along tubular tools whichsit at the head of the pedicle screw. Such a centring part 39 is shownwith its under end in FIG. 6. It has been screwed into the threadprovided for a clamping screw 6 and tapers from a cylindrical start viaa cone 40 up to a flexurally elastic central part 41. The cable 7 isalready fixed by a clamping screw 6 in FIG. 8. A tubular mating holderhaving an internally guided screwdriver which is described in a parallelapplication can be used for the fixing. Such a mating holder issupported in a shape matched manner in notches 10 of the head 4 and iscontinued upwardly in tubular shape. If this mating holder is left inposition after the screwing in of the clamping screw 6, it can be usedlike the centring part 39 for the guiding of a lateral projection 30with a guiding fork 35 on its way to the pedicle screw. The end 26 ismade to form a fork 42 which is centred at the drawn in cable 7. Thepre-tension to the pedicle screw is applied via pressure zones 38 whichare arranged at both sides of the cable 7 at the fork 42.

A hemispherical projection 36 is shown for the lateral projection 29 inFIG. 7 and can be centred in the passage bore 31 of a pedicle screw.Pressure zones 37, which can transfer a pre-tension force to the pediclescrew 3, are arranged at both sides of the hemispherical projection 36.

PARTS LIST

-   1 spinal vertebra-   2 spinal vertebra-   3 pedicle screw-   3′ pedicle screw-   4 head-   5 end face-   6 clamping screw-   7 cable-   8-   9 intervertebral disc-   10 notch-   11 limb-   12 limb-   13 axis of rotation-   14 key-   15 flexural spring-   16 pointer-   16 a pointer-   17 trailing pointer-   18 scale part-   19 screw-   20 handle-   21 projection-   21 a thumb grip-   22 scale (spacing)-   23 arrow marking-   24 scale (pre-tension)-   25 crossbar-   26 end-   27 end-   28 hook-   29 lateral projection-   30 lateral projection-   31 passage bore-   32 elongate slot-   33 groove-   34 guide fork-   35 guide fork-   36 hemispherical projection-   37 pressure zone-   38 pressure zone-   39 centring part-   40 cone-   41 flexurally elastic part-   42 fork-   43 rear part-   44 front part-   45 dog

1. A distance measuring instrument for pedicle screws, comprising firstand second limbs which cross at an axis of rotation and whose ends canbe positioned at two pedicle screws screwed into adjacent spinalvertebrae, with the first limb including a scale part at its oppositeend, the scale part being formed as a first pointer to display a spacingbetween two pedicle screws with the first pointer at a first scale ofthe scale part, wherein the second limb includes a second pointer and aspring and wherein the scale part is incorporated into a handleconfigured to produce a spreading force at the ends via the spring, thespreading force being readable relative to the second pointer via asecond scale.
 2. A distance measuring instrument in accordance withclaim 1, wherein the scale part includes an arc-shaped crossbar of thefirst limb.
 3. A distance measuring instrument in accordance with claim1, wherein the spring is a flexural spring.
 4. A distance measuringinstrument in accordance with claim 1, wherein the first limb includesan elongate slot, the second limb being guided relative to the firstlimb in the elongate slot.
 5. A distance measuring instrument inaccordance with claim 1, wherein the scale part includes a groove andthe second pointer includes a key defining a trailing pointer guidedalong the groove, the trailing pointer showing the spacing of the endsof the first and second limbs on the first scale, the key remaining inplace at a maximum spacing measured.
 6. A distance measuring instrumentin accordance with claim 1, wherein at least one of the ends of thefirst and second limbs is designed as a fork configured to be centeredat a band or cable drawn through a head of a pedicle screw.
 7. Adistance measuring instrument in accordance with claim 1, wherein atleast one of the ends of the first and second limbs includes ahemispherical projection configured to be centered at a passage bore ina head of a pedicle screw.
 8. A distance measuring instrument inaccordance with claim 1, wherein the first and second limbs definerespective first and second axes, the first and second limbs includingrespective first and second projections respectively laterally offsetrelative to the first and second axes.
 9. A distance measuringinstrument in accordance with claim 8, wherein the projections defineadapter pieces pushable onto the limbs.
 10. A distance measuringinstrument in accordance with claim 8, wherein each of the projectionsincludes a guide fork oriented in a spreading direction, the projectionsconfigured to guide the respective ends of the first and second limbs toa head of a pedicle screw along a centering part thereof screwed intothe head of the pedicle screw.
 11. A distance measuring instrument inaccordance with claim 1, wherein the second limb includes a thumb grip,the second scale being configured for reading of pretension and beingattached to a hook that engages behind the second pointer to preventoverstraining of the spring by the thumb grip.
 12. A distance measuringinstrument in accordance with claim 1, wherein the first limb includes acrossbar having a plastic, the first and second limbs including a metal.13. A method of measuring a distance between pedicle screws, comprising:providing a distance and force measuring instrument, the distance andforce measuring instrument comprising: first and second limbs whichcross at an axis of rotation, with the first limb including a scalepart, the scale part being formed as a first pointer to display aspacing with the first pointer at a first scale of the scale part,wherein the second limb includes a second pointer and a spring; andwherein the scale part is incorporated into a handle configured toproduce a spreading force via the spring, the spreading force beingreadable relative to the second pointer via a second scale; couplingends of the instrument to at least two pedicle screws; applying aspreading force to the pedicle screws via the spring of the handle;reading the spreading force indicated by the second pointer on thesecond scale; and reading the spacing indicated by the first pointer onthe first scale.
 14. A method in accordance with claim 13, furthercomprising: selecting an implant based on the spacing indicated by thefirst pointer on the first scale.
 15. A method in accordance with claim14, wherein the implant includes at least one of a cable and an elementfor bridging the distance between the pedicle screws.
 16. Anintervertebral implantation system comprising: first and second pediclescrews configured for implantation into adjacent vertebrae; a bridgingelement configured for coupling to the first and second pedicle screws;and an instrument for measuring a distance between the first and secondpedicle screws, the instrument including first and second limbs whichcross at an axis of rotation and whose ends can be positioned at thefirst and second pedicle screws screwed into the adjacent vertebrae,with the first limb including a scale part at its opposite end, thescale part being formed as a first pointer to display a spacing betweenthe pedicle screws with the first pointer at a first scale of the scalepart, wherein the second limb includes a second pointer and a spring andwherein the scale part is incorporated into a handle configured toproduce a spreading force at the ends via the spring, the spreadingforce being readable relative to the second pointer via a second scale.17. A system in accordance with claim 16, wherein the bridging elementis flexible.